The subscript 'i' refers to the component x,y, and z. It is an index that goes from 1 to 3, 1 = x, 2 = y, 3 = z that is used for the sum. In this case, it means the sum of [tex]mV_x A_x + mV_y A_y + mV_z A_z[/tex] in component terms.

In more advanced studies, you'll see the summation term dropped and when you see a repeated index such as the one you have, that kind of sum is implied. On the other hand, if you have something like [tex]A_i B_j[/tex] where A and B are just two indices, the index runs from 1 to 3 on both i and j so you get 9 terms that include [tex]A_1 B_3 , A_2 B_3 , A_1 B_1 [/tex] etc etc.